1. Field of the Invention
The present invention relates to an acoustic measuring apparatus and method, and to a program executed by the apparatus.
2. Description of the Related Art
For example, when audio signals reproduced by a multi-channel audio system are output from a plurality of loudspeakers and are listened to by a listener, a sound field (or sound radiation) perceived by the listener differs as the sound balance or the sound quality changes depending upon the listening environment, e.g., the structure of the listening room, the listening position of the listener with respect to the loudspeakers, etc. Under some conditions of the listening environment, the listener at the listening position may not perceive a desired sound field.
This problem is particularly critical in, for example, a vehicle cabin. In a vehicle cabin, a listener mostly sits on a seat, and the distance between the listener and individual loudspeakers varies. The difference in the arrival time of sounds from the loudspeakers causes a largely unbalanced sound field. Since the vehicle cabin is relatively small and substantially closed, a complex synthesized sound including reflection, etc., reaches the listener, and causes an unbalanced sound field. Due to the limitation of space in which the loudspeakers are installed, it is difficult to place the loudspeakers so that sound can reach the listener's ear directly from the loudspeakers. Variations in the sound quality affect the sound field.
An acoustic correction approach is common to allow the listener to listen to sound in a desired sound field similar to the actual sound source in a listening environment using an audio system. In the acoustic correction approach, for example, delay times of audio signals to be output from the loudspeakers are adjusted to correct for the difference in the arrival time of sounds at the listener's ear.
For more efficient acoustic correction, for example, it is desirable to automatically adjust the delay times using an acoustic correction apparatus rather than using only the auditory sensation of a user (or a listener).
Specifically, the acoustic correction apparatus first measures the acoustic characteristic of the listening environment, and sets acoustic-correction signal processing parameters of the sound output section in the audio system based on the measured acoustic characteristic. An audio signal processed according to these parameters is output from each loudspeaker, thus allowing the user to listen to the sound source in a desired sound field that has been corrected in accordance with the listening environment without adjusting the sound field.
In one known technique for measuring an acoustic characteristic and performing acoustic correction based on the measured acoustic characteristic, first, a microphone is placed at a listening position corresponding to the position of the listener's ear in the listening space. An acoustic correction apparatus outputs measurement sound from each loudspeaker. The output measurement signal is collected by the microphone to produce an audio signal, and the audio signal is analog-to-digital (A/D) converted. The acoustic correction apparatus obtains, for example, distance information between the individual loudspeakers and the listening position (i.e., the position of the microphone or the position at which sound is collected) based on the characteristic of the A/D converted measurement sound. Based on the distance information, sound-arrival time information in space from the individual loudspeakers to the listening position is obtained. The acoustic correction apparatus sets the delay time of a corresponding channel of audio signal to each loudspeaker using the sound-arrival time information about this loudspeaker so that the sounds output from the individual loudspeakers reach the listening position at the same time. Such correction is called time alignment.
Generally, a sine-wave signal or a burst signal is used as a measurement sound output from each loudspeaker to measure the distance between the loudspeaker and the microphone.
Japanese Unexamined Patent Application Publication No. 2000-261900 discloses an acoustic correction apparatus.